Page 253 - Adsorbents fundamentals and applications

238 CARBON NANOTUBES, PILLARED CLAYS, AND POLYMERIC RESINS

Pure metals (e.g., Fe, Co, or Ni) supported on alumina or silica gel lead to hol-
low graphite filaments with outside diameters around 0.1 µm. Higher dispersion
is needed for growing smaller filaments or MWNTs. This can be achieved by
using an additive to the metals, or by alloying the metals. Chen et al. (1997) were
able to grow uniform and small MWNTs on Ni-MgO from catalytic decomposi-
tion of methane or CO. The optimal starting catalyst was a mixed oxide prepared
◦
by co-precipitation to form Ni 0.4 Mg 0.6 O. The catalyst was heated in H 2 at 650 C
before switching to CH 4 for nanotube growth (Yang, 2000). The catalyst particle
was launched at the tip of the nanotube during its growth, and the catalyst could
be removed by dissolving in nitric acid solution. Prior to the nanotube growth
reaction, the catalyst was in the form of a solid solution of NiO and MgO, both
having the same rock-salt crystal structure and nearly the same lattice constant.
During H 2 pretreatment and nanotube growth, a small portion of the NiO was
0
reduced to Ni . The effect of MgO was to inhibit, but not completely stop, the
reduction of NiO. The small portion of reduced Ni metal formed small particles
or clusters on the surface, which yielded MWNTs with small diameters. Thus,
uniform-sized MWNTs with outside diameters of 15–20 nm were grown. CaO,
on the contrary, did not have the dispersion effect on Ni (Chen et al., 1997), and
yielded large MWNTs.
Using micrometer-size zeolite crystals as the support for MWNT growth did
not present as an appealing idea due to the low surface area of the exterior
surfaces on which the metal is supported. Surprisingly, they turned out to be
excellent supports for Co and Fe, without added metals (Hernadi et al., 1996).
Hernadi et al. (1996) compared a variety of supports for Co and Fe, including
SiO 2 , carbon, and different zeolites. Co supported on NaY resulted in the highest
yield (27–40% yield) of uniform-sized MWNTs, which also had the highest BET
2
surface area (653 m /g). The same group subsequently provided a recipe for the
best results [Colomer et al. (1998)]. In their recipe, acetylene was decomposed on
◦
Co (2.5 wt %)/NaY at 600 C. The zeolite support was removed by dissolution
with HF solution, and the amorphous carbon was removed by either permanganate
oxidation or air oxidation. This recipe has been widely used. Figure 9.7 shows a
TEM image of MWNTs that were grown by using their recipe (Colomer et al.,
1998). No study or discussion has been made on the mechanism of Co dispersion
on zeolite. The Y zeolite (i.e., faujacite) is in the form of cubic crystals with a
dimension in the order of a micrometer, with its pore size much smaller than
the particle size of the Co metal. The zeolite is first impregnated with a Co
salt (acetate) at room temperature, subjected to subsequent calcination. During
calcination, water vapor, CO 2 ,and CH 4 will evolve from the internal pores. This
may help the initial dispersion of Co. The external silicoaluminate surface is
apparently better than both alumina and silica for the dispersion of Co. This
phenomenon deserves study.
A direct comparison of the MWNTs prepared by the two different recipes
(Chen et al., 1997, and Colomer et al., 1998) is as follows. MWNTs prepared
2
from the former recipe gave a BET surface area of 155 m /g, with a pore-size
distribution from 2.5 to 30 nm (with a peak size at 2.9 nm) (Long and Yang,